Bulbar raphe neurones with projections to the trigeminal nucleus caudalis and the lumbar cord in the rat: a fluorescence double-labelling study.

Retrograde transport of the fluorescent dyes Evans blue and 4,6-diamidino-2-phenyl indole (DAPI) has been used to study projections from the medullary raphe nuclei to the trigeminal nucleus caudalis and to the dorsolateral quadrant of the thoracolumbar cord in the rat. The majority of projecting neurones were found in n. raphe magnus (NRM) and its ventrolateral extensions over the pyramids and inferior olive. Double labelling experiments showed that 94% of raphe-trigeminal neurones sent a collateral branch to the nucleus in the contralateral brainstem. Similarly, 92% of raphe-trigeminal neurones branched to supply the thoracolumbar cord. It is suggested that the widespread nature of the analgesia produced by electrical stimulation in NRM in conscious animals may be due to activation of a population of raphe neurones which inhibit the responses of neurones in the trigeminal nucleus and dorsal horn via an extensive system of collateral projections to these structures.